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Available online at www.sciencedirect.com

Separation and Purification Technology 61 (2008) 161–167

Selective adsorption of carbon dioxide, methane and ethane byporous clays heterostructures

Joao Pires a,∗, Maria Bestilleiro a, Moises Pinto a, Antonio Gil b

a Department of Chemistry and Biochemistry and CQB, Faculty of Sciences, Building C8, University of Lisbon, Campo Grande, Lisboa, Portugalb Department of Applied Chemistry, Los Acebos Building, Public University of Navarre, Campus of Arrosadia, E-31006 Pamplona, Spain

Received 13 April 2007; received in revised form 8 October 2007; accepted 11 October 2007

bstract

The separation of binary mixture representatives of natural and landfill gases by selective adsorption using adsorbent porous clay heterostructuresas been investigated. Using Wyoming clay as a starting material, the solids are prepared by the gallery-templated approach from the polymerizationf two silica sources, tetraethoxysilane (TEOS) and phenyltriethoxysilane (PhOS), at several molar ratios. In this way, the surface chemistry and theorosity characteristics of the samples are modified. The adsorbents presented specific surface areas up to 634 m2 g−1, micropore-size distributions

ith maxima near to 0.6 nm, thermal stability up to 400 ◦C and also hydrophobic characteristics. The selectivity for the separation of variousinary mixtures such as CO2/CH4, CO2/C2H6 and C2H6/CH4, estimated by a methodology based on the determination of the Gibbs free energy, isiscussed. 2007 Elsevier B.V. All rights reserved.

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eywords: Adsorption; Clays; Carbon dioxide; Methane; Ethane

. Introduction

The better energy use, with less economical and environ-ental impacts, is a major concern of the society in general and

rovides enduring challenges for scientists and engineers [1].he purification of natural gas, in general, and of landfill gases,

n particular, to the best use of methane as energy source is aelevant subject. Natural gas is mainly composed of methaneith small amounts of hydrocarbons and diluents, which varyy region and over time [2]. For some processes such as theatalytic combustion in natural gas fired turbines, which haseen demonstrated as an effective and economical technologyor the abatement of the emission of NOx pollutants in electricower generation, the purity and the relatively uniform compo-ition of the natural gas is very important [3]. In the case of these of methane from landfill gases, the main factor that affectshe feasibility of producing natural gas is the cost of small-scale

urification units. In order to be able to use landfill gases for nat-ral gas vehicle applications, or even for power, methane muste separated from the other components of the landfill gases

∗ Corresponding author. Tel.: +351 217 500 898; fax: +351 217 500 088.E-mail address: jpsilva@fc.ul.pt (J. Pires).

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383-5866/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.seppur.2007.10.007

o obtain a high purity product. Carbon dioxide in the landfillases can reach an amount of 40–45% and, therefore, is a majoromponent that has to be efficiently separated to improve theconomics of developing liquid, or eventually compressed, nat-ral gas. The separation of carbon dioxide, from methane andther gases, by processes that involve adsorption is consideredery promising [4–6] and several families of adsorbents haveeen studied and developed within this context [7–11].

Natural clays are low-cost materials that can be found in aumber of soils. Chemically, they are hydrous aluminosilicateshere, besides silicium and aluminum, other common species

re iron or magnesium. Clay mineral particles have dimensionsf less than 2 �m and can be classified in various groups [12,13].egarding their use as adsorbents, and because that by them-

elves, clays do not have permanent and definite porosity, thexpandable 2:1 group of clays [12,13] has been used to syn-hesize pillared interlayered clays (PILCs in short). These new

aterials have permanent porosity and the processes of pillar-ng using several methodologies were reviewed elsewhere [14].ypically the obtained materials have specific surface that can

each up to 350 m2 g−1 and some of these solids were appliedn various adsorption and separation processes [15,16].

Recently [16–20], porous clays heterostructures have beenrepared to enhance the thermal stability of PILCs in presence

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62 J. Pires et al. / Separation and Puri

f water. These materials are obtained by the polymeriza-ion of one (or two) silica sources, using the gallery templatepproach, producing materials with specific surface areashat are significantly higher than those obtained with PILCsnd can reach between 800 and 1000 m2 g−1. The moresual silica source is the tetraethoxysilane but other silanesuch as phenyl or methyltriethoxysilane were also studied20,21].

This work aims to study the possibilities for the use in sepa-ation and purification processes, which involve the enrichmentn methane of natural gas or from landfill gas components,f porous clays heterostructures. These materials are preparedrom a natural clay and two sources of silica, namely theetraethoxysilane and the phenyltriethoxysilane, as a method-logy to change the nature of the surface chemistry of theaterial.

. Experimental

.1. Materials

A smectite clay from Wyoming (Volclay SPV-200), obtainedrom the American Colloid Company, with the chemical compo-ition (Si3.91Al0.09)IV(Al1.51Fe0.18Mg0.26)(Ca/2, K, Na)0.49 [22]nd a cation exchange capacity of 115 mequiv./100 g [23] wassed as starting material.

The preparation method of the porous clays heterostructuresas based on the literature [16–20]. The first and the second

teps in the preparation of the materials were common to all therepared samples. The first step consisted in the intercalationf the clay with an ionic surfactant, to expand the clay layers.etyltrimethylammonium bromide, CTAB (Aldrich) was usednd 4.8 cm3 of a CTAB 0.5 mol dm−3 aqueous solutions weredded to a water clay suspension. The resulting mixture washen kept overnight at 50 ◦C, also under stirring, after whicht was centrifuged and washed with deionized water until neu-ral pH and air-dried. The second step in the preparation of theamples was the addition of a co-surfactant, that is, a neutralmine [18,19]. In the present work, and considering a previ-us study [16], octylamine was used. The addition of 4.3 cm3

f octylamine (Aldrich) was made under stirring, which wasept for 0.5 h. In the following, the silica source was added andhe resulting mixture stirred for 3 h. Two silica sources weresed: tetraethoxysilane, TEOS (Aldrich, 98%) and phenyltri-thoxysilane, PhOS (Fluka, 98%). The amounts of TEOS andhOS were adjusted in order to keep constant the total molarroportion in relation to the quantity of amine. A 1:6 molaratio, octylamine:(TEOS + PhOS), was used and various mate-ials were obtained with an increase in the amount of PhOSrom samples labeled from P1 to P3. In this way, the material1 was prepared only with TEOS (159 mmol), P2 with 145 and3.25 mmol of TEOS and PhOS, respectively and P3 with equal

mounts (79.5 mmol) of each silica source. The CTAB and thectylamine molecules have been removed of the porous struc-ure under reflux with a HCl solution (1 mol dm−3) in ethanol20,21]. The samples are finally dried in an oven at 100 ◦C.

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n Technology 61 (2008) 161–167

.2. Methods

X-ray diffractogram patterns were determined in a PhilipsX 1820 instrument using the Cu K� radiation. Oriented andon-oriented mounts [12] were used, and diffraction peaks wereetected. This was previously reported in the literature [16], andan be related to the poor long-range order presented by someypes of porous clays heterostructures [16,24].

The diffuse reflectance infrared Fourier transform (DRIFT)pectra of the samples were collected on a Nicolet 6700 atcm−1 resolution using the smart diffuse reflectance accessory,t room temperature with a DTGS TEC detector. The samplesere prepared by mixing with KBr in an agate mortar. Each

ollected spectrum was an average of 128 scans of the sampleubtracted by the average of 64 background scans using onlyBr in the sample container.Thermogravimetric and the heat-flow curves were obtained in

TG-DSC model 111 (Setaram, France) in a flow of dry nitrogenAir Liquide, 99.999%), from room temperature to 650 ◦C withheat rate of 5 ◦C min−1. Before the experiments, the samplesere maintained in a wet atmosphere, to assure the same initial

onditions of analysis for all the samples for, at least, 1 week.Nitrogen (Air Liquide, 99.999%) physisorption experiments

ave been performed at −196 ◦C using a static volumetric appa-atus (Micromeritics ASAP 2010 adsorption analyzer). Theamples, about 0.2 g each, have been previously degassed for4 h at 200 ◦C at a pressure lower than 0.133 Pa. The gas adsorp-ion data have been obtained by successive gas doses up to aelative pressure of 0.01. Each point of the adsorption isothermn this range has been equilibrated for at least 2 h in order toharacterize correctly the smallest micropores. Further gas haseen added and the volumes required to achieve a fixed set ofelative pressure have been measured.

The adsorption isotherms of carbon dioxide, ethane (Airiquide, 99.995%) and methane (Matheson, 99.995%), wereetermined in a conventional volumetric apparatus, with a pres-ure transducer (Datametrics, mod. 600) previously calibratedgainst a mercury manometer, equipped with a vacuum pumpystem which allowed a vacuum better than 10−2 Pa. The adsorp-ion temperature was maintained with a water bath (VWR) at5 ◦C. Before experiments, samples were previously degassedor 2.5 h at 200 ◦C. In the calculation of the adsorbed amounts,he non-ideality of the gas phase was taken into account whenetermining the adsorbed amounts, by using the second virialoefficients. Since the studied pressure range was until the atmo-pheric pressure, this correction was small, usually lower than.2%.

. Results and discussion

.1. DRIFT and thermal analysis

The DRIFT (Fig. 1) spectra were obtained in the range

etween 1300 and 3200 cm−1 to avoid the intense and broadbsorption band in the 1000–1300 cm−1 region, assigned to thesymmetric stretching modes of Si–O–Si vibrational moiety thatould faint the bands due to the aromatic ring of the phenyl

J. Pires et al. / Separation and Purifica

Fig. 1. DRIFT spectra between 1200 and 3200 cm−1.

Fh

gsaaCaaup

m(icBtsmttu

ciatpsastttomepaafhtbFopi[pr

3

paD

sctc−196 ◦C (0.81 g cm−3) [26]. The external surface area (Sext)

ig. 2. (a) Thermogravimetric (TG) curves (weight loss in percentage) and (b)eat-flow curves (HF) (in mW).

roup. The free phenyltriethoxysilane IR spectra [25] presentseveral absorption bands in the region of 1300–1500 cm−1,ttributable to the stretching of the C C double bonds in theromatic ring, and also between 2800 and 3100 cm−1 due to the–H bonds in the aromatic ring. As can be seen in Fig. 1, all thebove-mentioned bands are present in the DRIFT spectra of P2

nd P3 materials, the samples where phenyltriethoxysilane wassed, but not in the spectra of P1, confirming the presence of thehenyl groups in the former samples.

hmH

tion Technology 61 (2008) 161–167 163

The results of the thermal analysis are given in Fig. 2. All theaterials present a weight loss for temperatures until near 150 ◦C

see Fig. 2a). This weight loss is due to the physisorbed water ands more evident for P1 and P2, in line with the high adsorptionapacity of these materials when compared with P3 (see below).etween 150 and 400 ◦C the weight losses are minima, but above

his temperature a significant mass decrease is observed for theamples where PhOS was used (P2 and P3) but not for the P1aterial. Therefore, the mass loss above 400 ◦C is attributed to

he decomposition of the organic part of the materials, due tohe phenyl group of PhOS, meaning that these solids are stablentil 400 ◦C.

A first indication on the effect of the differences on the surfacehemistry of the prepared materials can be obtained consider-ng the slope of the TG curve between the ambient temperaturend 150 ◦C. The slope is higher for P2 than for P1, meaninghat water is more readily removed from the former as a mostrobable consequence of some degree of hydrophobicity in theample P2 due to the presence of the phenyl groups. Taking intoccount the heat-flow curves summarized in Fig. 2b, it can beeen that the temperature at the minima of the curves ascribedo the water loss, which is observed at 129 ◦C for P1, is shiftedo a lower temperature in the case of P2 (119 ◦C). The curve ofhe sample P3 shows the lowest value, at 104 ◦C. This sequencef temperatures should be related to the surface chemistry of theaterials, namely of its more hydrophobic nature due to the pres-

nce of the phenyl groups. It should be noticed, for comparisonurposes, that in the case of a highly hydrophilic material suchs the X-zeolite, the minimum in the heat-flow curve is obtainedt a temperature near to 200 ◦C (results not shown). This resultound is significantly higher than for the studied porous clayseterostructures. The TG-DSC results discussed above indicatehat the studied materials present two important characteristics toe used in adsorption processes that involve organic molecules.irstly, they can be considered hydrophobic and, the adsorptionf organic molecules will be in principle promoted even in theresence of some degree of humidity a situation already reportedn the literature for other types of porous clays heterostructures25]. Secondly, the materials are stable until relatively high tem-eratures, which gave them the possibility of being thermallyegenerated.

.2. Textural properties

The nitrogen adsorption of the samples, starting from lowressures, is presented in Fig. 3, where it can be indicated thatdsorption isotherms are of type I + II in the Brunauer, Deming,eming and Teller (BDDT) classification [26].The textural properties of the samples are more explicitly

ummarized in Table 1. The total pore volume (Vp) has beenalculated from the amount of nitrogen adsorbed at a rela-ive pressure of 0.99, assuming the density of the nitrogenondensed in the pores is equal to that of liquid nitrogen at

as been estimated by the t-plot method [26] and the specificicropore volume (V�p(HK)) has been calculated according theorvath–Kawazoe model [27]. As can be seen from this table,

164 J. Pires et al. / Separation and Purification Technology 61 (2008) 161–167

Table 1Textural properties derived from the nitrogen adsorption at −196 ◦C from various methodologies [27,36]

Sample ABETa (m2 g−1) Sext

b (m2 g−1) Vpc (cm3 g−1) Vmeso

d (cm3 g−1) Horvath–Kawazoe Dubinin–Astakhov

V�p(HK)e (cm3 g−1) dpHK

f (nm) V�p(DA)e (cm3 g−1) Eg (kJ mol−1) nh

P1 620 70 0.461 0.185 0.264 0.52 0.276 20.22 1.70P2 634 125 0.569 0.267 0.267 0.53 0.302 15.68 1.36P3 380 86 0.440 0.272 0.161 0.53 0.168 16.06 1.55

a Equivalent specific surface areas from the BET equation.b Specific external surface areas obtained from the t-plot method.c Specific total pore volumes at a relative pressure of 0.99.d Specific mesoporous volumes (from Vp − V�p(DA)).e Specific micropore volumes derived from the Horvath–Kawazoe (HK) and Dubinin–Astakhov (DA) methods.f Maxima of the Horvath–Kawazoe micropore-size distributions.g

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Characteristic energy from Dubinin–Astakhov equation.h Exponent of the Dubinin–Astakhov equation.

he sample P2 presents the highest value of the specific surfacerea, 634 m2 g−1, which decreases for the samples P1 and P3,20 and 380 m2 g−1, respectively. The three samples show sig-ificant amounts of micropores and mesopores, that is, poresith widths less than 2 and between 2 and 50 nm, respectively

28]. The analysis of the micropore-size distributions (MPSD)erived from the model proposed by Jaroniec–Gadkaree–Choma29] gave similar values for the maxima in these distribu-ions, for the three samples, which are within 0.58–0.63 nm.he MPSD obtained from this method has been included inig. 4.

.3. Adsorption of methane, ethane and carbon dioxide

The adsorption isotherms of methane, ethane and carbonioxide at 25 ◦C are presented in Fig. 5, where some trendsan be observed. Samples P1 and P2 always adsorb higher

mounts of any of the studied gases than P3, in line withreatest specific surface area and microporous volumes (seeable 1). The adsorbed amounts of the two hydrocarbons areigher in P2 than in P1, which still accords with the differ-

Fig. 3. Nitrogen adsorption at −196 ◦C.

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nces in their porous volumes, and equivalent specific surfacereas, but the reverse situation is verified for the adsorption ofarbon dioxide where the adsorbed amounts are higher for P1.he reason for this later observation is not clear but it is mostrobable a consequence of the interaction of carbon dioxide,hich has a quadrupole moment of 3.3 × 10−16 cm2 [30], with

he surface of the materials, which is enhanced in the case ofhe absence of the phenyl groups in P1. The initial adsorptionapacities of the samples were recovered after outgassing underacuum.

A methodology proposed by Myers [31] was used to estimatehe separation factors. Briefly, this methodology is based on theetermination of the equation of state for the Gibbs free energyf desorption of the solid adsorbent (the minimum isothermalork necessary to clean the adsorbent) at a given temperature

nd pressure, from the adsorption isotherms of pure gases. Thisequires an analytical expression for the adsorption isotherm. Inhe present work the virial equation in the form:

nadsads ads2 ads3

=

Kexp(C1n + C2n + C3n ) (1)

as used to fit the data, where K is the Henry constant, and1, C2, and C3 are the constants of the virial series expansion.

ig. 4. Micropore-size distributions derived from Jaroniec–Gadkaree–Chomaodel.

J. Pires et al. / Separation and Purification Technology 61 (2008) 161–167 165

Table 2Constants of the virial series expansion, Eq. (1)

Adsorbate Sample K (mol kg−1 kPa−1) C1 (kg mol−1) C2 (kg mol−1)2 C3 (kg mol−1)3

CH4 P1 2.33 × 10−3 −3.06 2.82 −1.21P2 2.59 × 10−3 −3.49 1.62 2.18P3 8.79 × 10−4 −7.40 11.88 0.00

C2H6 P1 1.15 × 10−2 0.24 −0.44 0.12P2 8.66 × 10−3 −0.42 −0.01 0.03P3 7.17 × 10−3 −0.25 −0.26 0.12

C −2

Tmtb

Ft

O2 P1 1.76 × 10P2 1.23 × 10−2

P3 4.23 × 10−3

he constants were calculated using the nonlinear least squaresethod and they are presented in Table 2. Eq. (1) has the advan-

age of having an asymptotic behavior at low pressures, and cane analytically integrated to obtain an expression for the free

ig. 5. Methane, ethane and carbon dioxide adsorption at 25 ◦C. The lines arehe results of the respective fittings of Eq. (1).

G

G

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0.61 −0.59 0.140.49 −0.96 0.38

−1.16 0.39 −0.07

ibbs energy:

= RT

∫ p

0

nads

pdp

= RT

(nads + 1

2C1n

ads2 + 2

3C2n

ads3 + 3

4C3n

ads4)

(2)

t should be emphasized that in Eq. (2) the adsorbed amount nads

nd the desorption Gibbs free energy G are functions of tempera-ure and pressure, i.e., n(T, p) and G(T, p). The expression in Eq.2) for G(T, p) can also be used to obtain other thermodynamicunctions, using fundamental thermodynamic relations [31].

Using the ideal adsorbed solution theory (IAST) [32], theomposition of the adsorbed and gas phases of mixtures ofhe studied gases (CH4, C2H6 and CO2) can be estimated. TheAST assumes that the mixing of the adsorbed phases of the twoomponents is ideal, although activity coefficients can also bestimated from adsorption isotherms of mixtures to account foron-ideal behavior [32,33]. Assuming the ideal behavior, at con-tant pressure and temperature, the difference in pure componentibbs free energy obeys the integral equation [34]:

2−G1=RT

∫ 1

0

[nads

1

y1−nads

2

y2

]dy1 ≈ nadsRT ln(S1,2) (3)

his equation can be used, at high pressure, to estimate theean selectivity (S) at constant pressure and temperature, by

n(S1,2) = (G2 − G1)/nadsRT , where G1 and G2 are the Gibbsree energy of components 1 and 2, in the same adsorbentolid, nads is the mean adsorbed amount, and y1 and y2 arehe molar fractions of components 1 and 2 in the gas phase.he constant pressure condition requires the evaluation of theure component adsorbed amounts (nads

1 , nads2 ), which satisfies

1(nads1 ) = p2(nads

2 ). This can be achieved by numerically solv-ng Eq. (1) for a given value of p. Using Eq. (2) the Gibbsree energies G1 and G2 can be calculated for the nads

1 , nads2 ,

espectively, and finally substituted in Eq. (3) to obtain the meanelectivity coefficients S [31,34].

The adsorption isotherms and fitted equations shown in Fig. 5ere used to estimate S as a function of p, using the above

escribed procedure. The results are presented in Fig. 6, for thehree possible mixtures on the three materials. It can be observedn this figure that the S values of the materials are relatively sim-lar for each mixture. Nevertheless, some important differences

166 J. Pires et al. / Separation and Purificatio

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psPatth

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4

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Cm

Fig. 6. Separation factors for the various systems.

an be noted. For the CO2/CH4 mixture the P1 material presentshe highest values of S, and the P2 and P3 are in the same range.or the C2H6/CH4 mixture, the P3 material presents the highesteparation factor, followed by P1 and P2, respectively. In thease of the CO2/C2H6 mixture, P1 and P2 have values closeo 1 (no effective separation) and higher than 1, contrary to P3hich presents values close to 1, but slightly lower than 1. This

esult means that C2H6 has higher affinity for the P3 surface thanO2, contrary to what happens in the P1 and P2 materials. Thisifference can also be observed comparing the C2H6/CH4 andO2/CH4 separation curves of the materials, since P3 showshigher separation factor for C2H6/CH4 than for CO2/CH4,

ontrary to the P1 and P2 materials. This is probably a conse-uence of aromatic rings that are present on the surface of the P3

aterial and promote highest affinity to C2H6 than to the CO2.s suggested above, the CO2 molecule has a strong quadrupoleoment and could interact more strongly with the OH groups of

he silica framework than with the phenyl groups. This should be

fTst

n Technology 61 (2008) 161–167

robably the fundamental reason for the observed differences,ince the P3 will have more phenyl groups at the surface than2 or P1, which have none. Additionally, the C2H6/CH4 sep-ration seems to be improved in the P3 sample, in relation tohe P1 and P2, indicating that the presence of phenyl groups onhe material surface enhances the adsorption interaction with theydrocarbon.

Comparing the values in Fig. 6 with the literature, the selec-ivity for CO2 in the CO2/CH4 mixtures found on the studiedaterials was similar to that reported by other authors on meso-

orous materials such as MCM-41 [35]. The selectivity for C2H6n the C2H6/CH4 mixtures is lower [7]. It is believed that porouslays heterostructures can be selective adsorbents of interest foreparations related with the improvement of the quality of natu-al and landfill gases, and with the abatement of carbon dioxide,or various reasons. Namely, the existence of a considerablemount of mesopores, which makes the structure less hinderedhen compared with microporous materials, and it is important

or separations based on equilibrium selectivity. Mesoporosity islso responsible for the fact that at atmospheric pressure the sat-ration capacities are far from being reached, as indicated by thehape of the adsorption isotherms in Fig. 5, which would allowhese materials to adsorb at high pressures. Considering the sur-ace chemistry of the studied materials, the hydrophobic naturef the surface it is also advantageous for working situationshere water vapour is also present. Eventually, more important

s the possibility of the tailoring of the surface chemistry by intro-ucing various organic groups, in order to change the adsorptionelectivity of these organic–inorganic nanocomposite materials.

. Conclusions

In this work, porous clays heterostructures were preparedrom a Wyoming clay and two types of silica sources, theetraethoxysilane and the phenyltriethoxysilane. The use of thewo silica sources allowed modifying the porosity and the sur-ace chemistry of the adsorbents. The materials presented micrond mesoporosity. The specific surface area, which reached upo 634 m2 g−1 for the sample where only tetraethoxysilane wassed, decreased with the increase in the proportion of phenyltri-thoxysilane. The samples were structurally stable until 400 ◦Cnd the surface presented hydrophobic properties. The amountsf adsorbed methane and ethane follow the trend of the porousolumes of each sample, but for carbon dioxide an inversionas found between the sample where only TEOS was used and

he sample prepared with the lowest proportion of PhOS. Thisbservation could be explained by the specific interaction of theO2 molecule and it is a consequence of the change of the sur-

ace chemistry of the materials by the TEOS/PhOS ratio used inhe synthesis.

The selectivity for the separation of the binary mixtures:O2/CH4, CO2/C2H6 and C2H6/CH4, was estimated using aethodology based on the determination of the equation of state

or the Gibbs free energy of desorption of the solid adsorbent.he highest separation factors were obtained for the CO2/CH4ystem when the samples synthesized with only TEOS or withhe lowest amount of PhOS are used as adsorbents. An inver-

rifica

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ng

R

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J. Pires et al. / Separation and Pu

ion in the sequence of the separation factors was found for theample prepared with the highest amount of PhOS. In this case,he most favorable separation was for the mixture C2H6/CH4.

cknowledgments

This work was partially funded by FCT through the plurian-ual program of CQB. M.L. Pinto thanks FCT for a post-docrant (SFRH/BPD/26559/2005).

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